With the increasing demand for optical network capacity, the service transmission rate is developing from 10 Gb/s rate of current networks to 40 Gb/s and even 100 Gb/s. In high-speed optical transmission systems, selection of an optical modulation format is critical in the whole system. The optical modulation format is directly correlated to the properties of the optical transmission system such as transmission performance, spectral efficiency, nonlinear tolerance, and dispersion tolerance.
As a novel modulation format, a Return to Zero-Differential Quadrature Phase Shift Keying (RZ-DQPSK) modulation format has a narrow spectral width, can effectively inhibit nonlinear effects of various optical fibers and improve the tolerance to chromatic dispersion and polarization mode dispersion, and becomes an important modulation mode for long-distance high-speed large-capacity optical transmission.
FIG. 1 shows a device for generating RZ-DQPSK optical signals in the prior art. As shown in FIG. 1, an RZ-DQPSK optical signal is generated through two stages of modulators. The first stage is a data modulator, including two differential Mach-Zender Modulators (MZMs) and a π/2 phase shifter; and the second stage is a clock modulator. FIG. 2 is a flow chart of generation of an RZ-DQPSK optical signal in the prior art, which includes the following steps. A first MZM is loaded with an NRZ signal 1 to generate a Non Return to Zero-Differential Phase Shift Keying (NRZ-DPSK) code 1, and similarly, a second MZM generates an NRZ-DPSK code 2. The NRZ-DPSK code 1 and the NRZ-DPSK code 2 pass through a π/2 phase shifter, and are combined into an NRZ-DQPSK optical signal. The clock modulator is loaded with a synchronous clock signal, and generates an NRZ pulse envelope as driven by the clock signal, and cut the pulse of the NRZ-DQPSK signal to generate an RZ-DQPSK optical signal. Another solution in the prior art is to reverse the order of the first stage modulator and the second stage modulator to generate an RZ-DQPSK optical signal.
In the implementation of the present invention, the inventor found that the prior art has at least the following problems.
In the prior art, the two stages of modulators are used to generate the RZ-DQPSK optical signal, which results in complex structure, high costs, and large volume of the device for generating optical signals. Meanwhile, the clock modulator increases the overall insertion loss (that is, a ratio of the output optical power to the input optical power) of the modulators, and in order to maintain the output optical power unchanged, the requirements for the input optical power are increased. The clock modulator needs to control a bias point, which increases the complexity of loop circuit control. In addition, the phase of the clock must be synchronous with that of the data signal, but the synchronization control method is complex.